Beam deflection devices are known in optics in a great variety of embodiments. Many beam deflection devices, in particular those in which rapid deflection of light beams is important, contain galvanometer mirrors. Among the disadvantages of beam deflection devices based on galvanometer mirrors are the large frictional losses in the ball bearings. As a consequence of its design, the return spring in the galvanometer positioning elements is not arranged symmetrically; with rapid oscillations in particular, this becomes perceptible as a difference in motion between the forward and return strokes of the galvanometer mirror.
Beam deflection devices that contain two mirrors arranged one after another, rotatable about mutually perpendicular rotation axes, are often used to deflect a light beam in two dimensions. In other beam deflection devices for two-dimensional deflection of a light beam, a single gimbal-mounted mirror is used. German Application DE 196 54 210 A1 discloses an optical arrangement for scanning a beam in two axes located substantially perpendicular to one another, in particular for use in confocal laser scanning microscopes. The optical arrangement contains two mirrors rotatable, each by means of a drive system, about mutually perpendicular axes; and a further mirror that is associated, nonrotatably in a defined angular position, with one of the two mirrors, so that the mirrors associated with one another rotate together about the Y axis and thus rotate the beam about a rotation point that lies on the rotation axis (X axis) of the third mirror, which rotates alone.
C. Cheever et al., “Ferrofluid Film Bearing for enhancement of rotary scanner performance”, SPIE vol. 1454 Beam Deflection and Scanning Technologies (1991), pp 135–151, discloses a mirror which can be rotated about one axis and which is mounted in a Ferrofluid Film bearing.
In scanning microscopy, a sample is illuminated with a light beam in order to observe the reflected or fluorescent light emitted from the sample. The focus of an illuminating light beam is moved in a specimen plane by means of a controllable beam deflection device, generally by tilting two mirrors, the deflection axes usually being perpendicular to one another so that one mirror deflects in the X direction and the other in the Y direction. Tilting of the mirrors is brought about, for example, by means of galvanometer positioning elements. The power level of the detected light coming from the specimen is measured as a function of the position of the scanning beam. The positioning elements are usually equipped with sensors to ascertain the present mirror position.
In confocal scanning microscopy specifically, a specimen is scanned in three dimensions with the focus of a light beam.
A confocal scanning microscope generally comprises a light source, a focusing optical system with which the light of the source is focused onto an aperture (called the “excitation pinhole”), a beam splitter, a beam deflection device for beam control, a microscope optical system, a detection pinhole, and the detectors for detecting the detected or fluorescent light. The illuminating light is often coupled in via the beam splitter, which can be embodied, for example, as a neutral beam splitter or a dichroic beam splitter. Neutral beam splitters have the disadvantage that depending on the splitting ratio, a great deal of excitation light or detected light is lost.
The fluorescent or reflected light coming from the specimen travels back via the beam deflection device to the beam splitter, traverses it, and is then focused onto the detection pinhole behind which the detectors are located. Detected light that does not derive directly from the focus region takes a different light path and does not pass through the detection pinhole, so that a point datum is obtained which results, by sequential scanning of the specimen, in a three-dimensional image. A three-dimensional image is usually achieved by acquiring image data in layers, the path of the scanning light beam on or in the specimen ideally describing a meander (scanning one line in the X direction at a constant Y position, then stopping the X scan and stewing by Y displacement to the next line to be scanned, then scanning that line in the negative X direction at constant Y position, etc.). To make possible acquisition of image data in layers, the sample stage or the objective is shifted after a layer is scanned, and the next layer to be scanned is thus brought into the focal plane of the objective.
It is particularly important in scanning microscopy, in order to achieve an aberration-free image, to rotate the illuminating light beam about a rotation point located in the objective pupil as the sample is being scanned. Ideally, this condition is met for scanning both the rows and the columns, i.e. for both the X and the Y direction. Since the beam deflection device usually cannot, for space reasons, be positioned directly in the objective pupil, the rotation point located in the pupil is imaged by way of further optical systems, and the beam deflection device is positioned at the location of the image. Beam deflection devices based on two mirrors arranged one after another have the disadvantage that only one of the mirrors can be positioned at the location of the pupil image. This necessarily results in an aberration. Beam deflection devices based on a single gimbal-mounted mirror, or the previously mentioned beam deflection device known from German Unexamined Application DE 196 54 210 A1, eliminate this disadvantage; because of the greater masses to be moved, these beam deflection devices permit only a lower scanning speed.